The present invention relates to a method and apparatus for coding/decoding a motion image signal.
A typical scheme for encoding a motion image signal by combining orthogonal transformation coding and interframe predictive coding is described in IEEE Transactions on Communications, Vol. COM-33, PP. 1291-1302, December 1985 (Reference 1). The configuration of a coder/decoder system described in Reference 1 is shown in FIG. 1. According to this system, a coder comprises an orthogonal transformation circuit 1, a subtracter 3, a quantizer 53, an adder 4, and a predictor 91, and a decoder comprises an adder 11, a predictor 92, and an inverse orthogonal transformation circuit 13. Reference numerals 100 and 300 denote input terminals; and 200 and 400, output terminals, respectively. According to this system, in the coder, an orthogonal transformation coefficient is coded according to interframe coding. In the decoder, the orthogonal transformation coefficient is decoded according to interframe decoding and is transformed according to inverse orthogonal transformation to obtain a decoded image. When an interframe difference of the orthogonal transformation coefficients is to be coded, all orthogonal transformation coefficients are quantized on the basis of identical quantization characteristics. The number of data to be generated is determined by changing a step size and a dead zone of the quantization characteristics.
In the 2nd International Technical Symposium on Optical and Electro Optical Applied Science and Engineering, SPIE Conf. B594, Image Coding, December 1985 (Reference 2), another conventional encoder/decoder system for performing orthogonal transformation coding of an interframe difference signal is described. The configuration of the encoder/decoder system in Reference 2 is shown in FIG. 2. In this encoder/decoder system, the encoder comprises a subtracter 51, an orthogonal transformation circuit 52, a quantizer 53, an inverse orthogonal transformation circuit 54, an adder 55, a frame memory 56, a predictor 57, a frame memory 58, a motion detector 59, and a multiplexer 60. The decoder of this system comprises a demultiplexer 61, an inverse orthogonal transformation circuit 62, an adder 63, a frame memory 64, and a predictor 65. Reference numerals 100 and 300 denote input terminals; and 200 and 400, output terminals, respectively. In the system shown in FIG. 2, the orthogonal transformation coefficients are divided into several groups which have variances, and the orthogonal transformation coefficients having small variances are omitted. Therefore, coding is performed at a predetermined bit rate while degradation of image quality of the decoded image signal is minimized. The scheme for coding the orthogonal transformation coefficients in FIG. 2 can be applied to the scheme for coding an interframe difference of the orthogonal transformation coefficients in FIG. 1.
FIGS. 3A and 3B respectively show two examples wherein the coding scheme for omitting the orthogonal transformation coefficients in FIG. 2 is used in the arrangement of Reference 1 of FIG. 1. More specifically, the coder in FIG. 3A performs coefficient omissions within the coding loop, while the coder in FIG. 3B performs coefficient omissions outside the coding loop. Each coder comprises an orthogonal transformation circuit 1, an interframe predictor 2, a subtracter 3, an adder 4, a coefficient omission circuit 5, and a coefficient omission judgement circuit 81. The coefficient omission judgement circuit 81 judges the magnitude of energy of the interframe difference signal and which coefficient is to be omitted. Reference numeral 100 denotes an input terminal of an image signal; 200, an output terminal of a coded image signal; and 210, an output terminal of mode information representing which coefficient is to be omitted. In the conventional systems described above, the following drawbacks are presented.
(1) In the system for coding orthogonal transformation coefficients according to interframe coding, when the number of coefficients to be omitted is increased, coding can be performed using a smaller number of data. When the coefficients are omitted, coding errors occur. However, if omissions are performed for the small coefficient values obtained by orthogonally transforming a current frame signal, coding errors are small. In the conventional system, coefficients to be omitted are determined by the magnitudes of the interframe differences. In the conventional system, therefore, if the orthogonal transformation coefficient of the current frame is small although that of the previous frame is large, the interframe difference is large, and these orthogonal transformation coefficients are not omitted but coded. More specifically, in the conventional system, although it is suitable to omit the small orthogonal transformation coefficient of the current frame and not to code it, the coefficient is coded since the interframe difference is large. Therefore, unnecessary coding is undesirably performed.
(2) In the conventional system, if an interframe prediction error is zero as a result of a coefficient omission, the coefficient of the previous frame is used as that of the current frame and is decoded in the decoder in the system. As a result, a signal waveform of the previous frame corresponding to the omitted coefficient is left in the decoded signal of the current frame. Image quality is greatly degraded. In particular, in a motion image portion, if coefficients having large interframe differences are omitted, a high-frequency component of the moving object, i.e., an edge is left in the background.
FIG. 4 is a view for explaining the above operation in correspondence with the arrangement of FIG. 1. FIG. 4 shows one-dimensional orthogonal transformation. Two waveforms in the block represent those of two of the orthogonal transformation coefficients. The amplitude of the waveform represents the magnitude of the coefficient. In this case, a DC component is not illustrated. Referring to FIG. 4, during coding of the previous frame, a locally decoded signal as in a waveform 508 is obtained. Assume that orthogonal transformation coefficients corresponding to waveforms 506 and 507 as components of the waveform 508 are sent to a decoder, and that a waveform 517 is obtained as a decoded signal. In a scheme for calculating a difference between the current and previous frame signals according to interframe coding, in the coder, the orthogonal transformation coefficients corresponding to the waveforms 506 and 507 are used as a current frame prediction signal output from the predictor 91 shown in FIG. 1. In the decoder, orthogonal transformation coefficients corresponding to waveforms 515 and 516 as components of a waveform 517 as an output from the predictor 92 in FIG. 1 are used as interframe prediction signals. When an input signal having a waveform 501 is to be coded in the current frame, an interframe difference between the orthogonal coefficients corresponding to the waveforms 502 and 503 as components of the waveform 501 and the orthogonal coefficients corresponding to the waveforms 506 and 507 is calculated. By this interframe difference calculation, orthogonal transformation coefficients corresponding to waveforms 504 and 505 are obtained. In the coefficient omission circuit 5, the orthogonal transformation coefficient corresponding to the waveform 504 is reserved, but the orthogonal transformation coefficient corresponding to the waveform 505 is omitted. As a result, the coefficients corresponding to waveforms 511 and 512 are coded. A circle in the coefficient omission circuit 5 represents preservation of the coefficient, and a cross represents omission of the coefficient. In the decoder, orthogonal transformation coefficients corresponding to waveforms 515 and 516 and predicted from the coefficient of the previous frame are output from the predictor 92 in FIG. 1, and orthogonal transformation coefficients of the current frame which correspond to waveforms 513 and 514 output from the predictor 92 in FIG. 1 are decoded. The orthogonal transformation coefficients corresponding to the waveforms 513 and 514 are subjected to inverse orthogonal transformation, thereby obtaining a decoded signal having a waveform 523. Orthogonal transformation coefficients used in prediction of a signal of the next frame are those corresponding to waveforms 509 and 510 in the coder and those corresponding to waveforms 518 and 519 in the decoder. These orthogonal transformation coefficients are input to the corresponding predictors 91 and 92 shown in FIG. 1. When the decoded waveform 523 is compared with the input waveform 501 and the decoded waveform 517 of the previous frame, the trailing edge of the ramp of the decoded waveform 523 is shifted to the right as compared with that of the input waveform and is similar to that of the waveform 517 of the previous frame.
(3) In Reference 2, image signals are divided into blocks, and interframe and intraframe coding schemes are employed in units of blocks. Although interframe coding is effective for a given orthogonal transformation coefficient in a given block since this coding scheme requires a small number of pixel data, another given orthogonal transformation coefficient in the given block may require intraframe coding in place of interframe coding. For example, assume that the frame is updated and an image pattern is slightly changed. In this case, interframe coding is effective for low orthogonal transformation coefficients since they are not greatly changed upon updating of the frame. However, high orthogonal transformation coefficients are greatly changed upon changing of the image pattern. In this case, intraframe coding may be better than interframe coding. In this manner, the amount of information cannot be greatly reduced according to the scheme in which the coding scheme is changed in units of blocks. According to a scheme using intraframe coding and interframe coding in units of orthogonal transformation coefficients, the number of data required for calculations can be reduced.
(4) According to the conventional schemes, omissions of the orthogonal transformation coefficients are determined according to the magnitudes of interframe difference signals. However, if man's sense of vision is taken into consideration, a resolution of the moving part can be low. By utilizing this phenomenon, the number of data can be further decreased.